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As i sat on the A2 and other parts of the local area in traffice for 2 1/2+ hours today i was wondering what was best to do. I was stationary a lot of the time for a long time and i kept switching my engine off (paranoid because of low fuel reading) while doing this i wondered if this was the best thing to do or not because i know i'm using the battery a lot by firing up the engine repeatedly.

You would be using more petrol by having to shoot a burst of petrol through to fire the enigne up each time u start it best be is to just let it idle it uses next to no petrol

SiDaBa wrote:As i sat on the A2 and other parts of the local area in traffice for 2 1/2+ hours today i was wondering what was best to do. I was stationary a lot of the time for a long time and i kept switching my engine off (paranoid because of low fuel reading) while doing this i wondered if this was the best thing to do or not because i know i'm using the battery a lot by firing up the engine repeatedly.

I seem to rember VW doing a Formel E which turned itself off in traffic and as soon as you went to engage gear it started as if by magic - i looked at a new one (when they were new and had one for a weekend test drive very good on fuel in town work - made no difference on a run!!!) When I asked the sales manager about the modifications to ensure starter motor/battery life - he just said its got a big battery and a heavy duty starter.

Anyway I digressed -

- Stop start work (short stops) I leave the engine running constant starting uses more fuel.......

- Traffic Jams/long periods of no movement I turn it off - long periods of idle uses fuel and pollutes the planet (*goes off to hug a tree*)

But then again

- I also turn my lights to sidelight when sitting at traffic lights at night so that they aren't glaring in someones rear view mirror

- I never sit in traffic with my brakes applied dazzling people behind - it why they fitted handbrakes

- I can't remember the last time I used fog lights at the front (no then again 1996 - I think), I uses the rears as necessary but turn them off as soon as someone comes up behind me

- I still double de-clutch when the engine is cold to give the syncro an easy time when gearbox oil is cold

All just things you get in the habit of doing

Ahem, the following rant may not be 100% based in fact.
But I sure would like to see some proof of the "its not worth it" argument vs what i can feel thru the car when driving it.



OK FOLKS

WHO TOLD YOU THIS "FACT"?? WHERE DID YOU LEARN IT?
Next time you'll be saying the world is flat. Come on! You think that we'd have hybrid cars, or damn even stuff like the Formel-E, if that was true?

1. It uses up a lot of battery to start the engine. No, it doesnt. You can start and stop your engine probably a hundred times before the battery is run out. Every tried cranking it over when there was a problem with it? I've had a half hour trying to get the polo working in winter and the battery was still fine at the end of it. Plenty of reserve - so long as you're running it for, say, 5 or 10 seconds in between, the alternator puts some juice back. Running for about a minute would totally refresh what was taken out by that one start. Think of how utterly, utterly knackered your battery is when it's run "flat" and needs a jump start, how quickly after that jump start you can actually turn it off and get it going again under it's own steam (answer: very quickly), but how long you have to drive it for before the battery is *fully* recovered (answer: an age!).... agh.
Sure if your battery is in a rough or run-down condition its a problem, but that's because you have a knackered battery!
You can get a polo engine going with a hefty kick down on the front edge of a driving wheel if it's jacked up and put in top gear after all, or a quick clutch-declutch in 3rd at any speed over 12mph. Unless it's winter, it doesnt need a very hard push in engineering terms. And after running for a few minutes in winter, it starts a lot easier anyway.
The formel-E had a heavy duty battery and starter, but that was back in 1983 when the battery was a lead-acid cell that needed regular topping up with distilled water, and replaced every 2 years, the engine and the (45w!) lights being about all that ran off it. You could rig it to run without one with a bit of knowhow. Nowadays its a much different story. Almost everything in a modern car is electric, and batteries are highly engineered sealed units that can go five years, maybe more. Don't know about the "heavy duty starter", but after nearly 80000 miles and quite a lot of motorway trekking, the little old electric dog has already put up with a lot of action and I bet it STILL has as much life left in it.

2. A "large amount of fuel" shoots through the engine when you're starting it up and this somehow negates the saving of knocking it off vs idling for an extended period? Just how big do you think your cylinders are? Particularly how much space is there for that liquid at TDC? And how rich a mix do you think it needs to get going before it all goes pear shaped and the spark is drowned out by too much fuel and not enough air? Divide the capacity at TDC by 13.5 for your ideal fuel:air mix, ie, 1/14th of that highly compressed capacity (say 140cc over all cylinders of a 1400cc car with 1:10 compression - or 10cc) is the maximum for any 2-revolution cycle of the engine. The car can happily idle with only a fraction of throttle and starts after, what, 3 or 4 revolutions? Even if it were maxed while cranking on the starter (which i doubt), its hardly an enormous amount of fuel compared to it whizzing away at 15 revs per second on idle. Even if also the reduced throttle opening cuts the effective compression back to 1:1...

My uncle spouts this one as well and the only basis I can think of it is borne out of the carbuerretor era when you very obviously had to send a lot of extra fuel thru the carb - using the choke - to keep the car running when cold. Maybe there's something intrinsic with carburettors as well, that they maybe leak fuel when starting, i dont know. But then again you can also start a warm carbed engine with no choke at all...
Modern cars simply dont need that or work on that principle and I bet not even the mk2s suffer from such a problem. The ECU delivers, via the injectors, as much fuel as is needed to create the optimum combustion at any time. It's not gonna drown the engine with petrol as that simply isnt necessary and is wasteful.

Idling in traffic you're still kicking out a good horsepower or so, and its not going anywhere. You can get your car moving along at up to about 20mph without touching the accelerator *at all* if you're careful with the clutch, and on a level surface it wont even rev up if you go from 3rd into neutral when idling along. There's a lot of energy and fuel going to waste there (take the bin-lid off an SPi and see how much gets sprayed) - tickover is a pretty inefficient state, besides the fact you're burning it and not going anywhere.
(vs that horsepower, the starter has maybe 400W (~1/2 hp) behind it, and can still both drag you along in 1st or 2nd gear AND start the engine)

Gah!!

I think this kind of thinking is why we choke in traffic jams, even in 15 mile tailbacks that havent moved for the last 10 minutes, and why hybrids are going to unintentionally save the planet.... by forcing the engines off when they're not being used. The Honda insight turned the town vs country economy figures on their head by getting 70mpg on the open road but something in excess of 100mpg in the city... which is the way it should be, as speeds are much lower - but so much fuel gets wasted by idling! (and is why a 1-litre that might not be inherently more efficient than a 1.3 at 70mph gets a better city score - it uses less to run per second at idle).

Yes. I do turn mine off in traffic. Routinely. And I mean any situation likely to lead to a wait of over 20 seconds, where it's not "waiting for a gap to turn right" but traffic lights, queue along a main road etc. Never have any trouble except when I cock it up myself.
The only thing I do worry about is my reaction time, the way my lights dim and the radio goes off when i turn the key, and if i've been standing a long time (like... more than 5 minutes), whether i can get away with a fast start seeing as the oil's probably sunk back into the sump.

Also if it's a traffic queue down a hill, i'll coast it's stop-starts on the handbrake. Oh so dangerous you say. What, at 5mph?

How about how much extra fuel it is to trot along at 35mph in 3rd rather than 4th? There's a lot more difference to be had there, anyway.


Thank you,
please.

A practical rule: if you're stopped for more than 2 1/2 - 3 minutes, turn the engine off and you'll be saving fuel. Anything less than that and you'll spend more fuel starting up (even with digital sequential multi-port fuel injection).

My Senator has a very accurate fuel computer and it indicates fuel usage as 0.3 gal per hour at tickover - keep with me here - it's a 3.0i and on a run will average over 36mpg with ease (long legged auto) lets say engine capacity and fuel usage is linear so I would hazzard a guess that a polo 1.4 would use 0.15 gal per hour at tick over.....

So two senarios (using 40mpg as the average consumption achieved by both cars in normal motoring)

Two cars economically driven for the same distance 80 miles with 1 hrs traffic jam unexpectedly encountered

Car A turns off whilst in traffic - Car B doesn't

Car A uses 2 gal of fuel (cos he turns off for the 1 hour) = 40 mpg

Car B uses 2 gal of fuel plus 0.15 gal for teh period stationary = 37.2 mpg

See even the maths say it's worth it

I don't know about fuel injected cars, but you should see the way my carb squirts petrol when you start it up... I tend to turn off the engine if I'm waiting more than a minute or so, but less than that I'll always leave it running because it really uses so little fuel idling. I do a lot of coasting down saffron walden high street when the traffic has backed right up to the top of the hill, because even if I have the engine running it's still not worth kicking it into gear to get moving so I tend to just turn it off all together. I never turn it off in any situation where I need to get going quickly, because it can be a bugger to start at times (need a new carb quite badly now, but I'm skint )

but mr bastard, you forget the "startup waste" (?!) factor, if it exists.

Tahrey1043 wrote:but mr bastard, you forget the "startup waste" (?!) factor, if it exists.

Yeah I conveinently forgot that mainly cos i don't think it makes a jacks sh1t worth of difference

You can call me Ian rather than MR B-Child - I know my father and he visited every day in the early hrs of the morning chink chink

I turn mine off because it annoys me. Although if you have a modified engine (e.g. remap, bigger injectors, high lift/duration cam etc) then your fuelling map will not cater for a low unladen tickover and will foul the spark plugs up, pretty much slowly flooding the engine. Although i say that, the rally cars i build still pass an sva on the rally ecu!

why shouldn't it?
i thought the point of having an ECU was that you could wring good power out of the motor whilst still having some low-end drivability/economy... rather than it being set up for one or the other with a carbed engine?

that's discounting wide angle cams, enormous intakes/exhaust etc of course.

edit: oops, you of course mentioned about that but i think it still holds for remaps - if you're chipping your car, you're unlikely to be wanting to break the landspeed record at 1500rpm..

(have vw tried any kind of VVT yet, to go with the FSI?)


"ian": badaboom-tshh